The present invention concerns a press installation for the separation of the liquid components from the solid components of a mixture of liquids and solids.
An installation for pressing of mixtures of liquids and solids is disclosed in our U.S. Pat. No. 4,429,628 granted Feb. 7, 1984. This installation includes pins protruding through the press cylinder in which a press screw is mounted. The pins point radially toward the screw and are intended to prevent the rotation of the material to be pressed with the screw. An adequate conveying pressure is built up by means of the pins.
Axial drainage bores are provided in the pins. The drainage bores open toward the axis of the screw and connect with drainage pipes for draining the separated liquid immediately at the location of its release, without producing a significant loss of pressure due to the drainage orifices.
As mentioned above, the pins prevent the rotation of the material with the screw, so that a high transport yield and thus high pressure in the area of the pin cylinder are produced. The high pressure also releases the intercellular and intervolume water of organic material. This water may be removed in view of the high pressure head of, for example, 300 bars in the press cylinder and 1 bar (atmospheric pressure) in the drainage bore holes.
As the distance between the tips of the pins and the base of the screw thread is relatively small, for example 0.3 mm, only very small solid particles will be entrained in the separated liquid. These particles are of such a size that they do not clog the drainage bore holes. The tips of the pins conform in shape to the curvature of the screw core.
It was found that, with the press installation according to our above mentioned patent, high transport rates and thus a high pressure can be obtained. The high pressure buildup is generated primarily by the drainage pins extending radially into the cylinder space. These pins prevent the rotation of the material to be dehydrated with the screw.
The liquid separated from the mixture is removed from the closed dehydration system at two locations:
(a) the liquid located on the inner wall is drained off through radial bore holes provided in the back of the screw thread, which in turn are connected with an axial screw bore hole in the core of the screw; and
(b) the separated liquid located in the base of the screw thread is removed by means of the axial drainage bores of the pins, with the pins terminating closely adjacent the core of the screw to form a parting gap. All drainage bore holes are connected with the drainage pipes.
It has now been determined that the transportation rate of the press installation is too high for certain materials, for example tree bark, and that consequently the dehydration retention time is too short, whereupon inadequately pressed material is discharged from the installation. It is therefore necessary to find ways to both extend the dehydrating retention time and also to increase the compression of the material in order to attain a higher degree of dehydration. This, however, should be accomplished without reducing the transportation rate, since this rate determines the productivity of the installation, and thus the economic viability of the system. As, however, the material compression depends to a great extent on the transportation rate, it is necessary to avoid a simple increase in the compression of the material.